The present invention relates to a method for forming an electric resistance welded steel pipe in which a flat skelp is continuously formed by rolls into a rounded skelp of which, after passing through fin pass rolls, both marginal edges are welded together into a finished steel pipe.
In the method for forming an electric resistance welded steel pipe having a seam line running in parallel with the longitudinal axis of the pipe, it is usual that a flat skelp trimmed in a width corresponding to the unfolded circumferential length of the desired pipe is first formed into a semicircular rounded skelp by successively passing through several forming rolls for the initial and intermediate forming stages followed by the stable forming of the edges through fin pass rolls at the finishing stage to finish the rounded skelp to have the predetermined shape and dimensions which is then subjected to the upset welding of the marginal edges to form the seam by means of a squeeze roll. Proper selection of the roll-forming process of the skelp before welding is very important because it is the determining factor for the quality of welding, yield in the forming, productivity and the operating rate of the mill, investment for the facilities, cost for rolls and the like.
The above described forming method of the electric resistance welded steel pipe is essentially a combination of the selected cross-sectional configuration of the skelp, i.e. flower pattern, in the forming process as a software-like technology and the actual selected type of the forming process in the forming facilities as a hardware. The flower pattern here implied means the patterns by which the successively changing cross sectional configuration of the skelp is expressed along the line of the sequence of the rolls at each of which the running skelp is deformed.
In respect of the flower pattern in the initial and intermediate forming stages in the prior art method, the flat skelp undergoes a bending formation over the whole width thereof with an object to reduce the working amount and forming load in the succeeding fin pass forming. The forming in these stages is of course performed in several different ways including the circular bending, combination of the circular bending and edge bending, combination of the W-bending and circular bending and combination of circular bending, edge bending and center bending. Each of these forming processes, however, has several problems such as the increased number of rolls in the initial and intermediate forming stages, increase in the length of the forming line, increased time taken for the adjustment and exchange of the rolls, decreased compatibility of the rolls with different pipe being formed, and the like.
The flower patterns in the above mentioned initial and intermediate forming stages may be of the center bending type which is, however, rarely undertaken in practice for the reasons described below and technically not well established. In the forming by center bending, the bending work of the skelp is started from the center line thereof toward the edge portions and the bending work at the initial and intermediate forming stages is limited to the center portion of the skelp and thereabout so that the bending work in these stages is easier than those in other forming processes. The marginal end portions of the skelp are, however, left unformed in the initial and intermediate forming stages so that the succeeding fin pass forming must accompany the edge bending which is a very difficult matter with increased load in the fin pass forming. Furthermore, the space-path length of the skelp edges in this case is the longest in comparison with other types of forming processes with increased appearance of the edge stretch and the resistance of the skelp against buckling is low because the edge portions have not yet been bent. Therefore, a disadvantage is sometimes unavoidable that edge wave frequently takes place between the roll stands.
In connection with the finish-forming stage, i.e. fin pass forming stage in the prior art method for forming an electric resistance welded steel pipe, two types of forming processes are known with different fin pass flower patterns. One is a forming process with the so-called Yoder-Mckay flower pattern in which the nearly rounded skelp is formed with successively decreasing width so as to increase the roundness of the bent skelp as a whole. The other is a forming process with the so-called Etna-type flower pattern in which the lower half of the skelp is bent in the initial and intermediate forming stages to have the final curvature of the finished pipe with the upper half bent to have 50 to 70% of the final curvature followed by the fin pass forming in which the upper half of the skelp is formed into the final curvature thus to form the marginal edge portions ready to be welded.
Accordingly, the conventional fin pass forming process can be considered as a reduce-type forming method mainly directed to the curvature increasing bending of the nearly rounded skelp.
In the above described Yoder-Mckay forming flower, each fin pass roll must serve to the bending of the rounded skelp over the whole width so that the load on the fin pass forming is relatively large. In addition, roll slip marks are sometimes unavoidable because the width decrease is effected on the side portions of the rounded skelp and the gap between the marginal edges of the rounded skelp is successively decreased resulting in a small edge gap after the final fin pass roll. Accordingly, the V-shape angle in the welding portion is too small to cause several problems including the occurrence of weld defects, generation of sparks at the V-shaped edge portion, appearance of edge waves and the like. A further problem is the increase in the thickness of the marginal edges due to the relatively large fin angle at the upstream-side fin pass rolls.
In the Etna-type flower pattern, on the other hand, the load in the fin pass forming is decreased and occurrence of the roll slip marks is reduced in comparison with the Yoder-Mckay flower pattern. In this case, however, difficulties are encountered in the forming at the initial and intermediate forming stages because the forming work in these stages must include bending over the whole width of the skelp or, in particular, finish bending of the skelp portion corresponding to the lower half of the semicircular bent skelp. Moreover, some of the problems in the Yoder-Mckay flower pattern remain unsolved in this case such as the narrowing of the V-shape angle at the welding portion as a result of narrowing of the edge gap after the final fin pass roll, the trend of the thickness increase at the marginal edges as a result of the increased fin angle at the upstream-side fin pass rolls and the like and the tendency of the thickness increase at the marginal edges is even larger than in the Yoder-Mckay flower pattern.
U.S. Pat. No. 4,339,938 discloses a rounded skelp forming method combined an edge crimping forming process with a sequential O-forming process thereafter. The edge crimping process comprises outermost edge bending and subsequent bending of the inward portion adjacent thereto by means of horizontal edge bend rolls. The circular O-forming process is performed by means of cage rolls disposed along the skelp. The method has such disadvantages as necessity in employing large breakdown rolls, occurrence of roll slip marks, and difficulties in integrated arc configuration in the upstream stages and in adjustment of separately disposed cage rolls.
Pipe manufacturing process composed of Uing, Oing and Expanding is well-known as the two dimensional discontinuous press forming process, which can not be utilized for the three-dimensional caliber roll pipe forming process because the former process is subjected to quite different plastic working technology from the latter with entirely different deforming behavior. In addition, UOE process can not be employed in complicated work such as over-bend forming and the like.
In short, the conventional methods practically performed for the forming of electric resistance welded steel pipes utilize the circular bend forming or a combination of the circular bend forming and another flower pattern as the type of the forming flower in the initial and intermediate forming stages and utilize the Yoder-Mckay or Etna-type flower pattern in the fin pass forming stage, while these forming flowers are actually obtained in a particular forming process such as the step-roll forming, vertical-roll forming, cage-roll forming and the like as is described later with reference to the accompanying drawings. These conventional processes, however, involve numbers of problems including; increase in the number of the forming roll stands as a result of the forming work by bending the skelp over the whole width in the initial and intermediate forming stages; decreased compatibility of the rolls with different kinds of pipes; increase in the length of time taken for the adjustment and replacement of rolls; unacceptable cross sectional form of the rounded skelp; increase in the load in the fin pass forming; narrowing of the V-shape angle at the welding portion as a result of the decrease in the final fin width; thickness increase in the edge portions of the skelp; necessity of adjustment of the fin pass reduction in accordance with the wall thickness or the grade of the product; increase in the residual stress in the circumferential direction of the rounded skelp; and so on.